Lightning Storm

The idea

A thunderstorm is a charge-separation engine. Inside the convective updraft, soft hail (graupel) collides with ice crystals millions of times over; the collisions transfer charge, the updraft hauls the light positively-charged crystals to the anvil while the heavier negative graupel settles low, and within minutes the cloud is a stacked capacitor with hundreds of millions of volts across it.

A flash is that capacitor finding a wire. First a stepped leader — a faint, branching channel of ionized air — gropes downward in ~50-metre jumps, which is where lightning's jagged shape comes from. When a branch connects (for cloud-to-ground strokes, usually to an upward streamer reaching from something tall), the circuit closes and the return stroke — the actual brilliant flash — races back up the channel at a third the speed of light. The channel briefly runs around 30,000 kelvin, five times the surface of the sun, and the shockwave of that heating is thunder.

Most lightning never touches the ground at all: flashes between and inside clouds outnumber cloud-to-ground strikes several to one. And because these clouds are rendered as genuinely dense participating media, a bolt behind or inside a cloud doesn't disappear — it lights the whole cloud from within. That diffuse glow is sheet lightning: ordinary lightning with the cloud as a lampshade.

What to look for

• Watch a strike develop in order: the leader draws its jagged, branching path first, then the return stroke floods the channel with light. The sequence is fast, but it is there.
• Sheet lightning: bolts buried in the deck light the cloud interior rather than showing a channel. The cloud's density is doing real optical work — nearer clouds occlude bolts behind them.
• In 'cloud-to-cloud', the arcs run long and horizontal, bowing and wandering between cells — these are the flashes that make a whole sky flicker without a single visible bolt.

Getting it right

• The bright flash travels upward. The downward leader is faint and slow; the return stroke that your eye registers as "the lightning bolt" runs ground-to-cloud.
• Thunder is not a separate phenomenon — it is the sound of the flash: explosive heating of the channel launching a shockwave that relaxes into rumble. The travel-time counting trick (three seconds per kilometre) works because light arrives effectively instantly.
• Lightning happily strikes the same place repeatedly — tall conductive points get hit precisely because leaders connect to upward streamers, and those launch from exactly such points.

Turn the knobs

• scenario picks the discharge geometry: 'cloud-to-ground' for plunging bolts, 'cloud-to-cloud' for the long horizontal arcs.
• exposure tunes the eye: low exposure isolates the bolts and their lit channels; high exposure brings up the cloud deck's ambient glow between strikes.